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        <h1 id="JS-运行机制"><a href="#JS-运行机制" class="headerlink" title="JS 运行机制"></a>JS 运行机制</h1><ul>
<li>面试时，关于<strong>同步和异步</strong>，可能会问以下问题：</li>
</ul>
<ol>
<li><p>同步和异步的区别是什么？分别举一个同步和异步的例子</p>
</li>
<li><p>一个关于 setTimeout 的笔试题</p>
</li>
<li><p>前端使用异步的场景哪些？</p>
</li>
</ol>
<ul>
<li>面试时，关于 <strong>js 运行机制</strong>，需要注意以下几个问题：</li>
</ul>
<ol>
<li><p>如何理解 JS 的<strong>单线程</strong></p>
</li>
<li><p>什么是<strong>任务队列</strong></p>
</li>
<li><p>什么是 <code>EventLoop</code></p>
</li>
<li><p>理解哪些语句会放入异步任务队列</p>
</li>
<li><p>理解语句放入异步任务队列的<strong>时机</strong></p>
</li>
</ol>
<h2 id="JS-的异步和单线程"><a href="#JS-的异步和单线程" class="headerlink" title="JS 的异步和单线程"></a>JS 的异步和单线程</h2><p>因为是单线程，所以必须异步。</p>
<p>我们通过题目来解释以下。</p>
<h3 id="题目一：异步"><a href="#题目一：异步" class="headerlink" title="题目一：异步"></a>题目一：异步</h3><p>现有如下代码：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(<span class="number">1</span>);</span><br><span class="line">setTimeout(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="number">2</span>);</span><br><span class="line">&#125;, <span class="number">1000</span>);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">3</span>);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">4</span>);</span><br></pre></td></tr></table></figure>
<p>上面的代码中，我们很容易知道，打印的顺序是 <code>1，3，4，2</code>。因为你会想到，要等一秒之后再打印 <code>2</code>。</p>
<p>可如果我把延时的时间从 <code>1000</code> 改成 <code>0</code>：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(<span class="number">1</span>);</span><br><span class="line">setTimeout(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="number">2</span>);</span><br><span class="line">&#125;, <span class="number">0</span>);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">3</span>);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">4</span>);</span><br></pre></td></tr></table></figure>
<p>上方代码中，打印的顺序仍然是<code>1，3，4，2</code>。这是为什么呢？我们来分析一下。</p>
<p><strong>总结</strong>：</p>
<p>js 是单线程（同一时间只能做一件事），而且有一个<strong>任务队列</strong>：全部的同步任务执行完毕后，再来执行异步任务。第一行代码和最后一行代码是同步任务；但是，<strong><code>setTimeout</code> 是异步任务</strong>。</p>
<p>于是，执行的顺序是：</p>
<ul>
<li><p>先执行同步任务 <code>console.log(1)</code></p>
</li>
<li><p>遇到异步任务 <code>setTimeout</code>，要<strong>挂起</strong></p>
</li>
<li><p>执行同步任务 <code>console.log(3)</code></p>
</li>
<li><p><strong>全部的同步任务执行完毕后，再来执行异步任务</strong> <code>console.log(2)</code>。</p>
</li>
</ul>
<p>很多人会把这个题目答错，这是因为他们不懂 js 的运行机制。</p>
<p>注意上面那句话：同步任务执行完毕后，再来执行异步任务。也就是说，<strong>如果同步任务没有执行完，异步任务是不会执行的</strong>。为了解释这句话，我们来看下面这个例子。</p>
<h3 id="题目二：异步"><a href="#题目二：异步" class="headerlink" title="题目二：异步"></a>题目二：异步</h3><p>现有如下代码：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(<span class="string">'A'</span>);</span><br><span class="line"><span class="keyword">while</span> (<span class="number">1</span>) &#123;</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">'B'</span>);</span><br></pre></td></tr></table></figure>
<p>我们很容易想到，上方代码的打印结果是 A，因为 while 是同步任务，代码会陷入死循环里出不来，自然也就无法打印 B。可如果我把代码改成下面的样子：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(<span class="string">'A'</span>);</span><br><span class="line"></span><br><span class="line">setTimeout(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'B'</span>);</span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line"><span class="keyword">while</span> (<span class="number">1</span>) &#123;</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>上方代码的打印结果仍然是 <code>A</code>。因为 while 是同步任务，setTimeout 是异步任务，所以还是那句话：<strong>如果同步任务没有执行完，队列里的异步任务是不会执行的。</strong></p>
<h3 id="题目三：同步"><a href="#题目三：同步" class="headerlink" title="题目三：同步"></a>题目三：同步</h3><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(<span class="string">'A'</span>);</span><br><span class="line"></span><br><span class="line">    alert(<span class="string">'haha'</span>); <span class="comment">//1秒之后点击确认</span></span><br><span class="line"></span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'B'</span>);</span><br></pre></td></tr></table></figure>
<p><code>alert</code> 函数是同步任务，我只有点击了确认，才会继续打印 <code>B</code>。</p>
<h3 id="同步和异步的对比-☆"><a href="#同步和异步的对比-☆" class="headerlink" title="同步和异步的对比 ☆"></a>同步和异步的对比 ☆</h3><p>我们在上面列举了异步和同步的例子。现在来描述一下区别：</p>
<p>因为 setTimeout 是<strong>异步任务</strong>，所以程序并不会卡在那里，而是继续向下执行（即使 settimeout 设置了倒计时一万秒）；但是 alert 函数是<strong>同步任务</strong>，程序会<strong>卡在那里</strong>，如果它没有执行，后面的也不会执行（卡在那里，自然也就造成了<strong>阻塞</strong>）。</p>
<h3 id="前端使用异步的场景"><a href="#前端使用异步的场景" class="headerlink" title="前端使用异步的场景"></a>前端使用异步的场景</h3><p>什么时候需要<strong>等待</strong>，就什么时候用异步。</p>
<ul>
<li><p>定时任务：setTimeout（定时炸弹）、setInterval（循环执行）</p>
</li>
<li><p>网络请求：ajax 请求、动态<code>&lt;img&gt;</code>加载</p>
</li>
<li><p>DOM 事件绑定（比如说，按钮绑定点击事件之后，用户爱点不点。我们不可能卡在按钮那里，什么都不做。所以，应该用异步）</p>
</li>
<li><p>ES6 中的 Promise</p>
</li>
</ul>
<p>代码举例：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(<span class="string">'start'</span>);</span><br><span class="line"><span class="keyword">var</span> img = <span class="built_in">document</span>.createElement(<span class="string">'img'</span>);</span><br><span class="line">img.onload = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'loaded'</span>);</span><br><span class="line">&#125;</span><br><span class="line">img.src = <span class="string">'/xxx.png'</span>;</span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">'end'</span>);</span><br></pre></td></tr></table></figure>
<p>先打印 <code>start</code>，然后执行 <code>img.src = &#39;/xxx.png&#39;</code>，然后打印 <code>end</code>，最后打印 <code>loaded</code>。</p>
<h2 id="任务队列和-Event-Loop（事件循环）"><a href="#任务队列和-Event-Loop（事件循环）" class="headerlink" title="任务队列和 Event Loop（事件循环）"></a>任务队列和 Event Loop（事件循环）</h2><h3 id="任务队列"><a href="#任务队列" class="headerlink" title="任务队列"></a>任务队列</h3><p>所有任务可以分成两种，一种是同步任务（synchronous），另一种是异步任务（asynchronous）。同步任务指的是，在主线程上排队执行的任务，只有前一个任务执行完毕，才能执行后一个任务。异步任务指的是，不进入主线程、而进入”任务队列”（task queue）的任务，只有”任务队列”通知主线程，某个异步任务可以执行了，该任务才会进入主线程执行。</p>
<p>总结：<strong>只要主线程空了，就会去读取”任务队列”，这就是 JavaScript 的运行机制。</strong>☆</p>
<h3 id="Event-Loop"><a href="#Event-Loop" class="headerlink" title="Event Loop"></a>Event Loop</h3><p>主线程从”任务队列”中读取事件，这个过程是循环不断的，所以整个的这种运行机制又称为 Event Loop（事件循环）。</p>
<p><img src="https://camo.githubusercontent.com/1b206c2f0184766d1aae57051f6d0d384656b8a5/687474703a2f2f696d672e736d79687661652e636f6d2f32303138303331305f313834302e706e67" alt="任务队列"></p>
<p>在理解 Event Loop 时，要理解两句话：</p>
<ul>
<li><p>理解哪些语句会放入异步任务队列</p>
</li>
<li><p>理解语句放入异步任务队列的时机</p>
</li>
</ul>
<h3 id="容易答错的题目"><a href="#容易答错的题目" class="headerlink" title="容易答错的题目"></a>容易答错的题目</h3><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i = <span class="number">0</span>; i &lt; <span class="number">3</span>; i++) &#123;</span><br><span class="line">    setTimeout(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(i);</span><br><span class="line">    &#125;, <span class="number">1000</span>);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>很多人以为上面的题目，答案是 <code>0,1,2,3</code>。其实，正确的答案是：<code>3,3,3,3</code>。</p>
<p>分析：for 循环是同步任务，setTimeout 是异步任务。for 循环每次遍历的时候，遇到 settimeout，就先暂留着，等同步任务全部执行完毕（此时，i 已经等于 3 了），再执行异步任务。</p>
<p>我们把上面的题目再加一行代码。最终代码如下：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i = <span class="number">0</span>; i &lt; <span class="number">3</span>; i++) &#123;</span><br><span class="line">    setTimeout(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(i);</span><br><span class="line">    &#125;, <span class="number">1000</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(i);</span><br></pre></td></tr></table></figure>
<p>如果我们约定，用箭头表示其前后的两次输出之间有 1 秒的时间间隔，而逗号表示其前后的两次输出之间的时间间隔可以忽略，代码实际运行的结果该如何描述？可能会有两种答案：</p>
<p>A. 60% 的人会描述为：3 -&gt; 3 -&gt; 3 -&gt; 3，即每个 3 之间都有 1 秒的时间间隔；</p>
<p>B. 40% 的人会描述为：3 -&gt; 3,3,3，即第 1 个 3 直接输出，1 秒之后，连续输出 3 个 3。</p>
<p>循环执行过程中，几乎同时设置了 3 个定时器，这些定时器都会在 1 秒之后触发，而循环完的输出是立即执行的，显而易见，正确的描述是 B。</p>

      
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        .css('overflow', 'hidden');
      $("#search-loading-icon").css('margin', '20% auto 0 auto').css('text-align', 'center');

      $.ajax({
        url: path,
        dataType: isXml ? "xml" : "json",
        async: true,
        success: function(res) {
          // get the contents from search data
          isfetched = true;
          $('.popup').detach().appendTo('.header-inner');
          var datas = isXml ? $("entry", res).map(function() {
            return {
              title: $("title", this).text(),
              content: $("content",this).text(),
              url: $("url" , this).text()
            };
          }).get() : res;
          var input = document.getElementById(search_id);
          var resultContent = document.getElementById(content_id);
          var inputEventFunction = function() {
            var searchText = input.value.trim().toLowerCase();
            var keywords = searchText.split(/[\s\-]+/);
            if (keywords.length > 1) {
              keywords.push(searchText);
            }
            var resultItems = [];
            if (searchText.length > 0) {
              // perform local searching
              datas.forEach(function(data) {
                var isMatch = false;
                var hitCount = 0;
                var searchTextCount = 0;
                var title = data.title.trim();
                var titleInLowerCase = title.toLowerCase();
                var content = data.content.trim().replace(/<[^>]+>/g,"");
                var contentInLowerCase = content.toLowerCase();
                var articleUrl = decodeURIComponent(data.url);
                var indexOfTitle = [];
                var indexOfContent = [];
                // only match articles with not empty titles
                if(title != '') {
                  keywords.forEach(function(keyword) {
                    function getIndexByWord(word, text, caseSensitive) {
                      var wordLen = word.length;
                      if (wordLen === 0) {
                        return [];
                      }
                      var startPosition = 0, position = [], index = [];
                      if (!caseSensitive) {
                        text = text.toLowerCase();
                        word = word.toLowerCase();
                      }
                      while ((position = text.indexOf(word, startPosition)) > -1) {
                        index.push({position: position, word: word});
                        startPosition = position + wordLen;
                      }
                      return index;
                    }

                    indexOfTitle = indexOfTitle.concat(getIndexByWord(keyword, titleInLowerCase, false));
                    indexOfContent = indexOfContent.concat(getIndexByWord(keyword, contentInLowerCase, false));
                  });
                  if (indexOfTitle.length > 0 || indexOfContent.length > 0) {
                    isMatch = true;
                    hitCount = indexOfTitle.length + indexOfContent.length;
                  }
                }

                // show search results

                if (isMatch) {
                  // sort index by position of keyword

                  [indexOfTitle, indexOfContent].forEach(function (index) {
                    index.sort(function (itemLeft, itemRight) {
                      if (itemRight.position !== itemLeft.position) {
                        return itemRight.position - itemLeft.position;
                      } else {
                        return itemLeft.word.length - itemRight.word.length;
                      }
                    });
                  });

                  // merge hits into slices

                  function mergeIntoSlice(text, start, end, index) {
                    var item = index[index.length - 1];
                    var position = item.position;
                    var word = item.word;
                    var hits = [];
                    var searchTextCountInSlice = 0;
                    while (position + word.length <= end && index.length != 0) {
                      if (word === searchText) {
                        searchTextCountInSlice++;
                      }
                      hits.push({position: position, length: word.length});
                      var wordEnd = position + word.length;

                      // move to next position of hit

                      index.pop();
                      while (index.length != 0) {
                        item = index[index.length - 1];
                        position = item.position;
                        word = item.word;
                        if (wordEnd > position) {
                          index.pop();
                        } else {
                          break;
                        }
                      }
                    }
                    searchTextCount += searchTextCountInSlice;
                    return {
                      hits: hits,
                      start: start,
                      end: end,
                      searchTextCount: searchTextCountInSlice
                    };
                  }

                  var slicesOfTitle = [];
                  if (indexOfTitle.length != 0) {
                    slicesOfTitle.push(mergeIntoSlice(title, 0, title.length, indexOfTitle));
                  }

                  var slicesOfContent = [];
                  while (indexOfContent.length != 0) {
                    var item = indexOfContent[indexOfContent.length - 1];
                    var position = item.position;
                    var word = item.word;
                    // cut out 100 characters
                    var start = position - 20;
                    var end = position + 80;
                    if(start < 0){
                      start = 0;
                    }
                    if (end < position + word.length) {
                      end = position + word.length;
                    }
                    if(end > content.length){
                      end = content.length;
                    }
                    slicesOfContent.push(mergeIntoSlice(content, start, end, indexOfContent));
                  }

                  // sort slices in content by search text's count and hits' count

                  slicesOfContent.sort(function (sliceLeft, sliceRight) {
                    if (sliceLeft.searchTextCount !== sliceRight.searchTextCount) {
                      return sliceRight.searchTextCount - sliceLeft.searchTextCount;
                    } else if (sliceLeft.hits.length !== sliceRight.hits.length) {
                      return sliceRight.hits.length - sliceLeft.hits.length;
                    } else {
                      return sliceLeft.start - sliceRight.start;
                    }
                  });

                  // select top N slices in content

                  var upperBound = parseInt('1');
                  if (upperBound >= 0) {
                    slicesOfContent = slicesOfContent.slice(0, upperBound);
                  }

                  // highlight title and content

                  function highlightKeyword(text, slice) {
                    var result = '';
                    var prevEnd = slice.start;
                    slice.hits.forEach(function (hit) {
                      result += text.substring(prevEnd, hit.position);
                      var end = hit.position + hit.length;
                      result += '<b class="search-keyword">' + text.substring(hit.position, end) + '</b>';
                      prevEnd = end;
                    });
                    result += text.substring(prevEnd, slice.end);
                    return result;
                  }

                  var resultItem = '';

                  if (slicesOfTitle.length != 0) {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + highlightKeyword(title, slicesOfTitle[0]) + "</a>";
                  } else {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + title + "</a>";
                  }

                  slicesOfContent.forEach(function (slice) {
                    resultItem += "<a href='" + articleUrl + "'>" +
                      "<p class=\"search-result\">" + highlightKeyword(content, slice) +
                      "...</p>" + "</a>";
                  });

                  resultItem += "</li>";
                  resultItems.push({
                    item: resultItem,
                    searchTextCount: searchTextCount,
                    hitCount: hitCount,
                    id: resultItems.length
                  });
                }
              })
            };
            if (keywords.length === 1 && keywords[0] === "") {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-search fa-5x" /></div>'
            } else if (resultItems.length === 0) {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-frown-o fa-5x" /></div>'
            } else {
              resultItems.sort(function (resultLeft, resultRight) {
                if (resultLeft.searchTextCount !== resultRight.searchTextCount) {
                  return resultRight.searchTextCount - resultLeft.searchTextCount;
                } else if (resultLeft.hitCount !== resultRight.hitCount) {
                  return resultRight.hitCount - resultLeft.hitCount;
                } else {
                  return resultRight.id - resultLeft.id;
                }
              });
              var searchResultList = '<ul class=\"search-result-list\">';
              resultItems.forEach(function (result) {
                searchResultList += result.item;
              })
              searchResultList += "</ul>";
              resultContent.innerHTML = searchResultList;
            }
          }

          if ('auto' === 'auto') {
            input.addEventListener('input', inputEventFunction);
          } else {
            $('.search-icon').click(inputEventFunction);
            input.addEventListener('keypress', function (event) {
              if (event.keyCode === 13) {
                inputEventFunction();
              }
            });
          }

          // remove loading animation
          $(".local-search-pop-overlay").remove();
          $('body').css('overflow', '');

          proceedsearch();
        }
      });
    }

    // handle and trigger popup window;
    $('.popup-trigger').click(function(e) {
      e.stopPropagation();
      if (isfetched === false) {
        searchFunc(path, 'local-search-input', 'local-search-result');
      } else {
        proceedsearch();
      };
    });

    $('.popup-btn-close').click(onPopupClose);
    $('.popup').click(function(e){
      e.stopPropagation();
    });
    $(document).on('keyup', function (event) {
      var shouldDismissSearchPopup = event.which === 27 &&
        $('.search-popup').is(':visible');
      if (shouldDismissSearchPopup) {
        onPopupClose();
      }
    });
  </script>





  

  

  

  
  
  
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  <script>
    
      pbOptions = {};
      
          pbOptions.iconStyle = "box";
      
          pbOptions.boxForm = "horizontal";
      
          pbOptions.position = "bottomCenter";
      
          pbOptions.networks = "Weibo,Wechat,Douban,QQZone,Twitter,Facebook,GooglePlus,Linkedin,Evernote";
      
      new needShareButton('#needsharebutton-postbottom', pbOptions);
    
    
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